Optimize list_refs usage by skipping unnecesary calls

What does this MR do and why?

Contributes to #556727 (closed)

Problem

New feature flag ref_existence_check_gitaly migrates reference existance checks from Redis to Gitaly. But some endpoints generate suboptimal chain of calls that lead to N+1 Gitaly calls.

Solution

• Skip the branch check for default branch
• Preload references for BranchesController

References

[FF] `ref_existence_check_gitaly` -- use Gitaly... (#556727 - closed)

How to set up and validate locally

1. Enable feature flag Feature.enable(:ref_existence_check_gitaly)
2. Visit http://gdk.test:3000/gitlab-org/gitlab-test/-/branches
3. You should see only one ListRefs call

Before	After

MR acceptance checklist

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Related to #556727 (closed)